Turbine blade for a gas turbine

ABSTRACT

There is described a turbine blade, with a platform and with an aerodynamically profiled blade leaf which extends transversely thereto and which comprises a suction-side blade leaf wall and a pressure-side blade leaf wall which extend from an inflow-side leading edge to an outflow-side trailing edge, with respect to a hot gas flowable along the platform or the blade leaf walls during operation, the platform and/or one of the two blade leaf walls having at least two adjacent regions. To prolong the service life of the turbine blade by equalizing the thermal stresses arising there during operation, the two regions have different heat transfer coefficients on the hot-gas side.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of European application No. 06010252.2EP filed May 18, 2006, which is incorporated by reference herein in itsentirety.

FIELD OF INVENTION

The invention relates to a turbine blade for a gas turbine, with aplatform and with an aerodynamically profiled blade leaf which extendstransversely thereto and which comprises a suction-side blade leaf walland a pressure-side blade leaf wall which extend from an inflow-sideleading edge to an outflow-side trailing edge, with respect to a hot gasflowable along the blade leaf walls during operation, the platformhaving at least two adjacent regions. The invention relates,furthermore, to a gas turbine having a turbine blade of this type.

BACKGROUND OF INVENTION

EP 1 469 163 A2 discloses, in this context, a cooled moving blade of agas turbine which inside it has cooling ducts running in ameander-shaped manner. Provided on the inner walls of the blade leafwhich delimit the cavities are turbulators which improve the heattransfer from the blade material into the coolant flowing through thecavity. Owing to the increased heat transfer, the turbine blade canpermanently withstand higher operating temperatures.

The disadvantage, in this case, is that cracks may occur, due toinadmissibly high temperature gradients, in the region of the flute-liketransition from platform to blade leaf, which is also designated as afillet, and/or in the platform. If the cracks which have occurredovershoot a critical crack length, then a reliable operation of the gasturbine equipped with such a turbine blade is not ensured.

Moreover, it is known from U.S. Pat. No. 4,023,350 to provide betweenthe supports of an exhaust gas duct of a turbomachine a multiplicity ofswirl generators in order to reduce the pressure loss which occurs andconsequently to increase the efficiency of the turbomachine.

SUMMARY OF INVENTION

A particularly long service life of the turbine blade is therefore adesign aim by which the availability of a gas turbine equipped with itcan be further increased. An object of the invention is, accordingly, toprovide a turbine blade for a gas turbine, the service life of which isprolonged.

The object directed at the turbine blade is achieved by means of ageneric turbine blade which is designed according to the features of theindependent claims. The invention is based on the knowledge that thewear and the occurrence of cracks and also the subsequent crack growthare thermally induced. The material of the turbine blade is exposed tothermal stresses, since substantially different material temperaturesarise in at least two regions of the turbine blade which are adjacent toone another. Conventionally, these regions are influenced by suitablemeasures, for example by cooling taking place inside the turbine blade,in such a way that these regions withstand the temperatures. However,since the cooling of the regions often takes place over a large area andtherefore cannot be adjusted to local requirements, for example onaccount of convective cooling, the regions subjected to a differentthermal load are cooled uniformly, thus giving rise to particularly hightemperature gradients in the blade material. However, these hightemperature gradients lead to the occurrence of cracks and crack growthwhich shorten the service life. It is proposed by the invention that thetwo material regions of the turbine blade which are adjacent to oneanother have different heat transfer coefficients on the hot-gas side(that is to say, occurring between the blade material and the hot gas),in order to equalize the thermal stresses occurring during operation inthe regions. Since the cold-side heat transfer coefficient (that is tosay, occurring between the blade material and the coolant) can sometimesbe set only with difficulty, the heat transfer coefficient on thehot-gas side is set for the first time in order to equalize thetemperatures in the blade material. The local thermal stresses occurringbetween the two regions can thereby be reduced substantially. In orderto achieve this, in a generic turbine blade, there is provision for aturbulence element arranged on the hot-gas side at the edge of theplatform to be arranged in one of the regions in order to set the heattransfer coefficient on the hot-gas side there. Owing to the equalizedthermal stress between the two adjacent regions, cracks therefore occurmore rarely than hitherto. And even if cracks should occur, their growthwill take place only more slowly, as compared with a turbine blade knownfrom the prior art. Correspondingly, by virtue of the invention, aparticularly long-life turbine blade can be specified, by means of whichthe period of availability of a gas turbine equipped with it is alsoincreased further. In particular, by virtue of the proposed measure, thefatigue lifetime (low cycle fatigue=LCF) for the platform and itstransition into the blade leaf, that is to say in the fillet, isprolonged.

Contrary to the common view that the introduction of heat from the hotgas into the blade material should always be kept as low as possible,the measure proposed here leads to an increased introduction of heat onaccount of an increased turbulence in the flow, with the result that thedifference of the material temperatures of the higher-loaded region andthe lower-loaded region is reduced. Due to the reduced difference in thematerial temperatures, a lower thermal stress occurs between the tworegions, thus achieving an equalization of the material temperaturewhich has the effect on the turbine blade of a prolonged service life.

Moreover, since cracks also occur at the edge of the platform, theturbulence elements are also provided at this location.

Advantageous refinements are specified in the dependent claims.

It has proved advantageous that that region of the platform which isadjacent to the suction-side blade leaf wall has the turbulenceelements. Preferably, the turbulence elements are provided in the middleregion of the platform which lies between the leading edge and trailingedge, as seen in the flow direction of the hot gas. Particularly in thissuction-side region of the platform, comparatively high temperaturegradients along the platform occur, due to the fillet enriched with moremass and to the usually convectively cooled platform or blade leaf, andare conducive to defects, such as the occurrence of cracks and crackgrowth. The provision of the means according to the invention in thisregion is accordingly of particular advantage.

Turbulators, dimples, ribs or pins are employed as turbulence elements.These known designs serve for inciting the turbulence of the hot gasflowing past, in order thereby to increase the heat transfer coefficienton the hot-gas side.

Instead of turbulence elements, the platform may also have a wavysurface as means for setting different heat transfer coefficients, thewave front of the wave form being oriented transversely, preferablyperpendicularly, to the flow direction of the hot gas. The wave crestsof the wave form serve in this case for the slight increase inturbulence in the hot gas flow, with the result that the heat transfercoefficient on the surface on the hot-gas side rises slightly. Aslightly slowed flow occurs in the wave troughs of the wave form, withthe result that the heat transfer coefficient falls slightly at thislocation. The wave troughs and the wave crests are arranged in regionsin which different thermal stresses have hitherto occurred.Consequently, even with a preferably slightly wave-like surface, anequalization of the thermal stresses arising during operation can beachieved.

Preferably, the platform has a first transverse edge onto which the hotgas is capable of flowing and a second transverse edge lying oppositethe first transverse edge, the platform having only two concave wavetroughs along the longitudinal extent of the platform between the firstand second transverse edge.

The proposed measures have proved to be particularly efficient when theturbine blade has been produced by a casting method, that is to say iscast, and when the blade leaf and/or the platform can be cooled,preferably can be cooled convectively. In particular, convectivelycooled turbine blades experience comparatively uniform cooling along thecooling duct in which the coolant, mostly cooling air, flows.Accordingly, the adaption of the heat transfer coefficient on thehot-gas side is particularly appropriate here. Expediently, the turbineblade is also free of any heat insulation layer and is thereforedesigned to withstand material temperatures of 850° C. to 1000° C. Thesetemperatures normally arise in the second or third turbine stage of astationary gas turbine used for current generation.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features may be gathered from the followingdescription of exemplary embodiments. Elements remaining essentially thesame are designated by the same reference symbols. Furthermore, asregards identical features and functions, reference is made to thedescription of the exemplary embodiment. In the drawing:

FIG. 1 shows a perspective illustration of a turbine moving blade,

FIG. 2 shows a sectional top view of a turbine moving blade,

FIG. 3 shows the temperature profile along the platform of the turbineblade,

FIG. 4 shows the adapted heat transfer coefficient α along the platformof the turbine blade, and

FIG. 5 shows the wavy platform in cross section according to thesectional line V-V.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows a perspective illustration of a turbine blade 10, designedas a moving blade, which has a fastening foot 12 of hammer-shaped crosssection for reception in a groove, not illustrated, of the rotor disk ofthe rotor of a gas turbine. The fastening foot 12 has adjoining it aplatform 14 which delimits the flow duct of the turbine radially, thatis to say transversely to the direction of the Z-axis. Provided on thesurface 16 of the platform 14 is a blade leaf 18 which extendstransversely to the platform 14 and which comprises a suction-side bladeleaf wall 20 and a pressure-side blade leaf wall 22 which extend from aninflow-side leading edge 24 to an outflow-side trailing edge 26. Inflowside and outflow side in this context relate in each case to the hot gas28 which flows through the turbine during operation and which flowsaround the blade leaf 18 essentially in the axial direction X. Theturbine blade 10 is uncoated, that is to say it has no heat insulationlayer, and is intended for use in the second or third turbine stage ofthe stationary gas turbine.

The blade leaf 18 of the turbine blade 10 is designed to be partiallyhollow and has two cavities 32 which are separated by a supporting rib30 and through which a coolant, preferably cooling air 36, supplied onthe fastening side can flow in parallel in the radial direction Z.

While the preferably stationary gas turbine is in operation, the bladeleaf 18 of a generic turbine blade heats up to approximately 850° C. to1000° C., and it is cooler, in particular, at its trailing edge 26 nearthe platform 14 in a pressure-side region 38 than in the oppositesuction-side region 40. In the turbine blade known from the prior art,the region 38 is, as a rule, more than 130° C. cooler than the region40.

Since the cast turbine blade thus has two regions 38, 40 which areadjacent to one another and are loaded differently during operation,according to the invention, to equalize the thermal stress on thesuction-side region 38 at the trailing edge 26, turbulators 44 areprovided which increase the heat transfer coefficient α on the hot-gasside with respect to the other of the two regions 40, as a result ofwhich the introduction of heat from the hot gas 28 into the bladematerial is also increased, contrary to the otherwise customary efforts.On account of the higher introduction of heat, in this region 38, theblade material is hotter than without the arrangement of turbulators 44.However, the admissible material temperature is in this case notovershot. Since the opposite region 40 on the suction-side blade leafwall 20 is in any case subject to substantially higher load duringoperation, that is to say, as a rule, is more than 130° C. hotter,without the presence of the turbulators 44 too great a temperaturedifference would occur between the two regions 38, 40 in the bladematerial, which would keep the thermal stresses at these locations at aninadmissibly high level, insofar as this is not effectively counteractedby the measure according to the invention. The turbulators 44 providedon the pressure side in the near-platform region near the trailing edge26 improve the heat transfer from the hot gas 28 into the bladematerial, so that, in the turbine blade 10 according to the invention,the difference between the temperature in the suction-side bladematerial and in the pressure-side blade material is equalized in such away that a difference of less than 100° C. can be achieved. On accountof the reduced temperature gradients, correspondingly lower thermalstresses arise, so that the two regions 38, 40 remain permanently freeof critical and lifetime-shortening fatigue phenomena, such as cracks.

The turbulence elements 42 may be designed as turbulators 44, dimples,ribs or pins and have been co-manufactured directly during the castingof the turbine blade 10. Turbulators 44 may be designed both asrib-shaped ribs, that is to say ribs running straight essentially alongtheir longitudinal extent, or as sickle-shaped ribs.

FIG. 2 shows a cross section through the blade leaf 18 of the turbineblade 10 as a top view, the blade leaf 18 in this case having fourcavities 32 through which cooling air 36 can flow sequentially. Thermalstresses arise in the material of the platform 14 on account of the hotgas 28 flowing along it and are dependent on the suction-side width, asseen in the circumferential direction Y, of the platform 14 between theedge 50 of the platform and the suction-side blade leaf wall 20. In afirst region A, the suction-side width of the platform 14 is greaterthan it is in a second region B. In a third region C, the suction-sidewidth between the platform edge 50 and the blade leaf 18 increasesagain. In these regions, because of the cooling of the blade leaf 18,different thermal gradients arise which could hitherto have led todefects. In particular, the region B has hitherto been affected by crackgrowth. In order to reduce the temperature gradients in the bladematerial, in particular in the region B, turbulence elements 42 in theform of turbulators 44 are provided locally in the surface 16 of theplatform 14 and increase the introduction of heat from the hot gas 28flowing past them into the turbine blade material. In this case, theturbulence elements 42 are arranged one behind the other with respect tothe flow direction of the hot gas, in order to adapt thermally aparticularly large area. The temperature difference between the firstregion A or the third region C, which have hitherto in any case beensubjected to higher thermal load, and the region B, which has hithertobeen subjected to lower thermal load, can be reduced significantly, withthe result that, overall, the thermal stress between the regions A, B, Cis equalized. The occurrence of cracks and crack growth can beeffectively avoided, thus resulting in a prolonged service life for theturbine blade 10.

FIG. 3 shows the temperature profile T in the platform material 14 alongthe axial direction X. The temperature T_(A) of the region A of theinflow-side transverse edge 52 is comparatively high, for example 850°C., and decreases in the direction of the hot gas 28 flowing along to atemperature minimum T_(B) which is to be found in the region B. Fromthere on, the material temperature rises again to a mean temperaturevalue T_(C) which occurs during operation in the region of theoutflow-side transverse edge 54 of the platform 14. The temperatures maybe measured by means of a suitable measurement method or be determinedsimulatively with the aid of a finite element computing program. Thetemperature difference between the region A and the region B hashitherto been of an order of magnitude higher than 130° C. This resultsin a temperature gradient over a distance of approximately 10 cm whichleads to thermal stresses in the blade material and may be conducive tocrack growth. By turbulence elements 42 being arranged in the region B,the difference of the temperature T_(A) and T_(B) was reduced to a valueof below 100° C., so that the thermally induced stresses could bereduced to an extent such that the occurrence of cracks and crack growtharise only with a delay or not at all.

FIG. 4 shows the heat transfer coefficient α as a function of theX-axis. The heat transfer coefficient α on the hot-gas side is higher inthe region B than in the regions A and C which are to be found at theinlet-side end 52 of the platform 14 and at the outlet-side end 54 ofthe platform 14. This wave-like characteristic curve of the heattransfer coefficient α along the platform 14 is attributable to theturbulence elements 42 which are provided in the region B for equalizingthe material temperatures of the platform 14.

FIG. 5 shows, in an alternative embodiment, the surface 16 of theplatform 14 along the sectional line V-V of FIG. 2. Instead ofturbulence elements 42, the surface 16 has a wavy design, so that, asseen in the radial direction Z, the height of the platform is increasedin the region B, as compared with the regions A and C. Thus, between thewave troughs A and C, a maximum B is provided which likewise leads to aheat transfer coefficient α adapted as required. The wave front of thewave-shaped surface 16 of the platform 14 may run transversely to theflow direction of the hot gas 28 or else perpendicularly to the platformedge 50.

Although the turbulence elements 42 cause slight aerodynamic flow lossesin the hot gas 28, the service life of the turbine blade 10 according tothe invention can be prolonged significantly, as compared with a genericturbine blade, since adaption of the heat transfer coefficient α on thehot-gas side for equalizing the thermal stresses and materialtemperatures is provided in one of at least two hitherto differentlyloaded thermal regions.

1.-8. (canceled)
 9. A turbine blade, comprising: a platform; anaerodynamically profiled blade leaf extending transversely to theplatform; a suction-side blade leaf wall extending from an inflow-sideleading edge to an outflow-side trailing edge; a pressure-side bladeleaf wall extending from the inflow-side leading edge to theoutflow-side trailing edge; at least two regions at the turbine bladewhich are adjacent in a flow direction of hot gas; and a turbulenceelement to equalize thermal stresses in the adjacent regions duringoperation of the turbine blade, wherein the adjacent regions are ofdifferent heat transfer coefficients on the hot gas side based upon theturbulence element.
 10. The turbine blade as claimed in claim 9, whereinthe turbulence element is arranged in one of the regions on the hot-gasside at an edge of the platform.
 11. The turbine blade as claimed inclaim 9, wherein an inflow and an outflow are based upon a hot gasflowing along the platform or the blade leaf walls during operation. 12.The turbine blade as claimed in claim 9, wherein the region of theplatform adjacent to the suction-side blade leaf wall has at least oneturbulence element.
 13. The turbine blade as claimed in claim 12,wherein the turbulence elements are in a middle region of the platformbetween the leading edge and the trailing edge.
 14. The turbine blade asclaimed in claim 9, wherein the turbulence element is a turbulator. 15.The turbine blade as claimed in claim 9, wherein the turbulence elementis a dimple.
 16. The turbine blade as claimed in claim 9, wherein theturbulence element is a rib.
 17. The turbine blade as claimed in claim9, wherein the turbulence element is a pin.
 18. The turbine blade asclaimed in claim 9, wherein a plurality of turbulence elements arearranged one behind the other in a flow direction of a hot gas.
 19. Theturbine blade as claimed in claim 9, wherein the turbine blade is freeof a heat insulation layer.
 20. A cast turbine blade, comprising: acooled platform; an aerodynamically profiled cooled blade leaf extendingtransversely to the platform; a suction-side blade leaf wall extendingfrom an inflow-side leading edge to an outflow-side trailing edge; apressure-side blade leaf wall extending from the inflow-side leadingedge to the outflow-side trailing edge; at least two regions of theturbine blade which are adjacent in a flow direction of hot gas duringoperation of the turbine blade, wherein the platform comprises theregions; and a turbulence element to equalize thermal stresses in theadjacent regions during operation, wherein the adjacent regions are ofdifferent heat transfer coefficients on the hot gas side based upon theturbulence element.
 21. The turbine blade as claimed in claim 20,wherein a plurality of turbulence elements are arranged one behind theother in a flow direction of a hot gas.
 22. The turbine blade as claimedin claim 21, wherein the turbulence element is a dimple or a pin.
 23. Agas turbine, comprising: a plurality of turbine blades, wherein theblades have: a platform with a wavy surface, an aerodynamically profiledblade leaf extending transversely to the platform, a suction-side bladeleaf wall, and a pressure-side blade leaf wall.
 24. The gas turbine asclaimed in claim 23, wherein a wave front of the wave surface isoriented transversely to a flow direction of a hot gas during operation.25. The gas turbine as claimed in claim 23, wherein wave troughs andwave crests are arranged in regions or close to regions of thermalstresses to influence the heat transfer coefficient.